Add support for 'make coverage' and --enable-coverage
testing: Testing and debug tweaks

 * We expect NFS mount for testing on target machine mounted
   in same place.
 * Documented in HACKING
 * Wait for daemon to exit properly for each test.
 * Fix up logging.
manager: Fix watching udisks2 for block object changes

This is because GDBusObjectManager is a real awkward API to use.
One has to listen to both objects being added and their interfaces,
the former being a figment of gdbus's imagination, no such thing
exists in DBus ObjectManager.
test-block: Complete basic block object testing
physicalvolume: Fix interface that UlPhysicalVolume implements
test-lvm: Add initial LVM test using vgcreate